1. Field of the Invention
The system of the present invention relates to distillation systems. More particularly, the present invention relates to a distillation system utilizing a loose polymer lining in a distillation vessel, piping and exchangers so that fluids may be distilled under vacuum, and the lining maintained against the wall of the vessel through a pressure differential from a second vacuum created around the outer lining wall.
2. General Background
The development of loose polymer-lined steel vessels, piping and exchangers has provided the chemical industry with valuable chemically inert materials of construction with improved economics over glass lined and exotic metal equipment, which are the alternatives for processing highly corrosive or very reactive compounds. The most widely used polymers are polyethylene, polypropylene, rubber and polytetrafluoroethylene (PTFE) more commonly known as TEFLON, which is a registered trademark of Dupont. TEFLON lined equipment is generally preferable over other polymers due to the much higher temperature limits and almost complete inertness of TEFLON to other chemicals. The exception to this inertness would be hydroflouric acid and metallic sodium. The typical type metals which would be required to handle similar operations are metals such as tantalum, titanium and zirconium. Nickel and chromium are also utilized. Glass-lined steel equipment is probably the oldest alternative but does not provide the full range of chemical inertness. Glass lined vessels are also very fragile requiring high costs of maintenance.
In the utilization of a TEFLON-lined vessel, loose polymer-lined steel utilizes a weep hole somewhere in the steel jacket for allowing entrapped air to escape, which would place the polymer-liner under atmospheric pressure or approximately 15 psi. Without the weep hole, entrapped air could expand and collapse the lining resulting in flow restriction or stoppage. The liner would generally tear and therefore would have to be replaced when collapse occurs.
A similar problem would develop when the interior liner pressure is reduced to sub-atmospheric levels. The liner would pulsate and collapse as vacuum is increased inside the liner. As operating temperatures are increased, the liner's physical strength deteriorates rapidly and collapse occurs at much smaller pressure differentials. As a consequence, the use of loose polymer lined steel equipment has not been possible in systems operated under vacuum. There is the ability to firmly bind the polymer liner to steel vessels so that operation in the vacuum is possible, but not suitable for high temperature uses, and in a very limited area.
An additional value of loose-lined polymer systems capable of vacuum operations lies in the ability to process chemicals at the much lower temperatures that vacuum distillation would allow. This capability would broadly increase the range of use for TEFLON-lined equipment since it is temperature limited in the range of under 430.degree. F. A typical example in common practice would be the ability to concentrate sulfuric acid to 93% in TEFLON lined equipment. The atmospheric boiling point is 530.degree. F. whereas the boiling point at full vacuum is under 270.degree. F. Therefore, in boiling under full vacuum, the distillation and reconcentration of the sulfuric acid could take place in the polymer lined vessel which would be beneficial due to its inertness, and ability to withstand this temperature. Several patents were found in the art which addressed the question of fluid sampling apparatus's having a lining, the most pertinent being as follows:
______________________________________ PATENT NO. TITLE INVENTOR ______________________________________ 2,223,785 GAS SAMPLING SYSTEM G. L. Hassler 2,645,940 SNAP SAMPLER J. Kohl et al. 3,635,092 MANUALLY OPERATED Maughan et al. GAS SAMPLER 3,866,474 GAS SAMPLE COLLECT- Hasselmann ING DEVICE 4,008,621 METHOD AND APPARA- Ostojic et al. TUS FOR SAMPLING GAS 4,635,487 FLUID SAMPLER Gowing ______________________________________